1. Field of the Invention
The invention relates to a continuous process for preparing alkylamines from alkyl halides by reaction with ammonia under pressure.
2. Background Art
Specific alkylamines, for example 3-(triethoxysilyl)-propylamine, are of great economic interest for a variety of fields. They are used, inter alia, as adhesion promoters in casting technology and the glass fiber industry, but also as crosslinkers. A further example is provided by hexamethylenediamine which is required in large volume for preparing polyamides.
The majority of patent publications are concerned with the batchwise preparation of alkylamines from alkyl halide precursors. For example, U.S. Pat. No. 4,234,502 describes the preparation of 3-(alkoxysilyl)propyl-substituted primary and secondary amines.
EP-A-849271 relates to a continuous process for preparing 3-(trialkoxysilyl)propylamines. In this process, 3-(trialkoxysilyl)propyl chloride is mixed with ammonia in the desired ratio and the mixture heated to the reaction temperature. The mixture passes through a pressure reactor, which may optionally comprise one or more temperature zones, with a residence time sufficient for complete conversion. Particular emphasis is given to a xe2x80x9ccriticalxe2x80x9d temperature of 110xc2x0 C. Below this temperature, 3-(trialkoxysilyl)propylamine hydrochloride is formed, which is dissociated by ammonia on heating to above 110xc2x0 C. to release free amine and ammonium chloride. The complicated workup is effected first by cooling the reaction mixture to separate a liquid silane phase. Separation of the silane phase has to be forced in some cases by adding a solvent. After separating the organic phase from the ammonium chloride-containing ammonia phase by means of extractors (mixer-settlers), the organic phase is processed distillatively. To remove the ammonium chloride, a substream of ammonia phase is depressurized, the ammonia released is condensed again, compressed, and recycled into the process, and the precipitated salt is isolated.
The known solubilities of ammonium halides in liquid ammonia may, depending on the temperature, be up to 80% by weight (iodide) at 55xc2x0 C., and even at xe2x88x9240xc2x0 C. are at least 10% by weight (chloride). Substantial separation of ammonium halides dissolved in ammonia without complete evaporation of the solvent is therefore very difficult.
It is accordingly an object of the invention to provide an improved continuous process for preparing aminoalkyl compounds which allows selective and continuous separation of the individual reaction products from the reaction mixture without adding extraneous materials. These and other objects are accomplished by the process of the subject invention.
The invention provides a continuous process for preparing alkylamines in which continuous streams of ammonia and alkyl halide in a molar ratio of at least 10:1 are reacted in a pressure reactor. The ultimate reaction mixture has a temperature of  greater than 80xc2x0 C., a pressure of  greater than 40 bar and an ammonium halide content of  greater than 1% by weight, and comprises two phases:
(A) a first phase which comprises at least 75% by weight of the total amount of ammonium halide formed, and
(B) a second phase which comprises at least 80% by weight of the total amount of alkylamine formed, which are then separated.
The process is based on the surprising discovery that the ultimate reaction mixture (the reaction mixture after the reaction has ended), when at a temperature of  greater than 80xc2x0 C., a pressure of  greater than 40 bar and an ammonium halide content of  greater than 1% by weight, separates into a previously undescribed liquid phase (A) which comprises at least 75% by weight of the total amount of ammonium halide formed and at most 20% by weight of the total amount of alkylamine and ammonia formed. The ammonia phase (B) which forms at the same time, accordingly comprises at most 25% by weight of the total amount of ammonium halide and at least 80% by weight of the total amount of alkylamine. The ammonia phase (B) has a markedly lower density than the ammonium halide phase (A).
The process allows selective and continuous separation of the individual reaction products from the end reaction mixture without adding extraneous materials and at the same time ensures that only a small portion of the process ammonia has to be completely evaporated and recondensed. The process allows high product yields and purities to be ensured at low preparation costs.
The (A) and (B) phases may be separated from each other using common liquid-liquid separation methods by utilizing the difference in density between the phases. The pressure in the separation is preferably at least 80 and at most 400 bar. Particular preference is given to separation taking place above the critical point of ammonia (132.4xc2x0 C., 112.8 bar), since under these conditions, the ammonium halide phase (A) comprises at least 85% of the total amount of ammonium halide formed and at most 10% by weight of the total amount of alkylamine formed. The ammonium halide phase (A) may optionally be extracted with pure ammonia during the separation in order to remove any residual amount of alkylamine.
Preference is given to continuously discharging phase (A), which still comprises up to 20% by weight of the total amount of alkylamine, from the process with depressurization, and preferably evaporating and recycling the ammonia present in this phase. The ammonium halide occurring in crystalline form may optionally be freed of any remaining alkylamine by washing with an organic solvent.
The ammonia phase (B), freed from the ammonium halide phase (A) and laden with the majority of alkylamine and small amounts of ammonium halide, is preferably depressurized at a lower pressure of at least 15 bar at a temperature of at least 50xc2x0 C., to separate a further fluid phase (C). The liquid phase (C) comprises a large proportion of alkylamine, and small amounts of ammonium halide and ammonia, and, owing to a marked density difference, may be separated by common methods from a less dense ammonia phase (D). Preference is given to compressing the separated, pure ammonia phase (D) and recycling it into the process. After discharging the alkylamine phase (C) with depressurization, it is preferably separated continuously by rectification into its components.
The molar ratio of ammonia to alkyl halide in the reaction is preferably at least 20:1. The molar ratio of ammonia to alkyl halide is preferably at most 150:1.
Preference is given to preheating the streams of ammonia and alkyl halide before entry into the pressure reactor. The reaction temperature is preferably at least 100xc2x0 C., more preferably at least 120xc2x0 C., and preferably at most 400xc2x0 C., more preferably at most 300xc2x0 C.
Particular preference is given to carrying out the reaction in supercritical ammonia, i.e. above the critical point (132.4xc2x0 C., 112.8 bar). In addition to the higher temperatures at these conditions, the low viscosity coefficients and high diffusion coefficients of the supercritical medium compared to liquid ammonia also have a positive effect on the reaction rate and selectivity.
In the process, preference is given to preparing aminoalkylsilanes from the corresponding alkyl chlorides. Preferred aminoalkylsilanes are of the general formula 1
(RO)3-nR1nSiR2NH2xe2x80x83xe2x80x83(1),
where
R is an optionally fluorine-substituted alkyl or alkoxyalkyl radical having from 1 to 6 carbon atoms,
R1 is an optionally fluorine-substituted hydrocarbon radical having from 1 to 12 carbon atoms,
R2 is an optionally fluorine-substituted alkylene radical having from 1 to 20 carbon atoms in which nonadjacent methylene units may be replaced by xe2x80x94Oxe2x80x94 groups and
n has the value 0, 1, 2 or 3.
R is preferably methyl, ethyl or propyl.
The R1 radicals are preferably unsubstituted. R1 is preferably a hydrocarbon radical having from 1 to 6 carbon atoms, in particular methyl, ethyl, propyl, vinyl or phenyl. The R2 radicals are also preferably unsubstituted. R2 is preferably an alkylene radical having from 1 to 6 carbon atoms, in particular methylene, ethylene or propylene.
All the symbols in the above formulae are each defined independently. In all formulae, the silicon atom is tetravalent. Terms such as xe2x80x9coptionally substitutedxe2x80x9d mean substituted or unsubstituted.